Process for removing metallic coatings from metallic parts



Patented May 13, 1941 UNITED STATES PATENT OFFICE PROCESS FOR. REMOVING METALLIC COAT- INGS FROM METALLIC PARTS William E. Day, Jr-, Plainfield, N. J., assignor to Mack Manufacturing Corporation, Long Island City, N. Y., a corporation of Delaware N Drawing. Application December 7, 1938, Serial No. 2443552,

Claims.

This invention relates toprocesses and to elec-- trolytic baths for removing metallic coatings, such as for example, copper, zinc or cadmium plate, from metallic parts, and more particularly relates to processes and electrolytes for removing residual copper coatings from heat-treated parts preliminary to the removal of fire scale which is formed on the parts by the heat treatment.

In many types of heat treating operations, it

is customary to. prevent hardening of portions of the parts by applying a thin copper plate to these portions. treatments cause the formation of fire scale on the unprotected portions which must be removed in order to provide a clean surface sothat the parts may be subsequently finished. Many types of scale removing treatments, such as, for example, the Bullard-Dunn process, which is disclosed in the Dunn Patents Nos. 1,775,671, 1,867,527, 1,898,765, 1,917,022 and the Fink and Wilber Patent No. 1,927,116 are highly satisfactory. The Bullard-Dunn process efiiciently removes fire scale and at the same time prevents corrosion of the parts by the pickling solutions which are later used to remove grease and other foreign substances. However, the presence of copper on the parts has been found to seriously afiect the efficiency of this process and it is necessary, therefore, to completely remove the copper-plate prior to the de-scaling treatment.

Heretofore, the copper has been electrolytically removed, with the aid of a cyanide or chromic acid bath. The cyanide bath treatment is efficient but is quite expensive and very dangerous because cyanide fumes are evolved. The chromic acid bath is likewise dangerous, expensive, and in addition must be very carefully controlled in Case hardening or cyaniding subject the operator to the effect of evolved poisonous gases.

Other objects of the invention will become apparent from the following description of typical processes and electrolytic baths for practicing the invention.

The invention consists generally of forming an electrolytic bath from ammonium hydroxide and ammonium nitrate and using the plated parts as anodes in conjunction with iron or steel cathodes in the electrolytic bath. The bath may suitably consist of an aqueous solution of ammonium nitrate and ammonium hydroxide, the ammonium nitrate and ammonium hydroxide bath a being superior to baths containing other nitrate salts. The process consists more particularly of using the plated parts as the anode and iron or steel scrap as a cathode and passing current through the electrolytic bath between the anode and the cathode ata current density between 20 to 30 amperes per square foot of anode.

In removing copper from copper-plated parts, the electrolyte is placed in a tank formed of materials not attacked by the electrolyte, for example, a ceramic tank, and the copper-plated parts are suspended in the tank. .Also suspended in the tank are pieces of steel or iron which form the cathode and which are connected together with the anode in an electric circuit.

When electric current is passed between the anode and the cathode through the electrolytic bath, the ammonium hydroxide and nitrate salt dissociate and the N03 and OH ions migrate toward the anode. The ammonium ions migrate toward the cathode where some of the ammonium ions may be initially lost as ammonia. The nitrate and hydroxide ions react. with the copper, dissolving it and forming complex copper order to avoid etching or corrosion of the metalthecopper from heat-treated parts having copper-plated portions which utilizes chemicals which will neither corrode metallic parts nor ammonium nitrate Cu (NHa)4(NO3)2. The copper ammonium nitrate migrates towards the cathode where it partially dissociates with the copper being plated out on the cathode. Inasmuch as the copper is not completely plated out, the bath gradually becomes saturated with the copper. The bath may be readily cleared, however, by replacing the copper-containing anode with an iron or steel anode and continuing to plate the solution. This causes a deposition of the copper on the steel or iron cathode and eventually causes the bath to clear.

A suitable bath may be formed by adding about 5% to 10% by volume of about 26 Baum ammonium hydroxide to an aqueous solution containing about 12% to15% by weight of ammonium nitrate. A, bath containing about 15% Electrolytic baths embodying the present in vention may be used in a similar way for removing zinc or cadmium plate from metallic parts.

Electrolytes of the type described above are substantially entirely stable, in fact, such baths have been used for long periods of time without requiring the addition of the salt or ammonium hydroxide. The stability of the bath is such as to render unnecessary the provision of an enclosure or a hood for the tank for drawing off the vapors. It appears that ammonia is' not evolved after the bath is placed in operation, for the reason that it is combined in the form of relatively stable, complex ammonium compounds and therefore little, if any, decrease in the con centration of ammonium hydroxide by the evolution of free ammonia can occur.

While the preferred forms of the invention have been described above, it should be understood that variations in the process and the electrolytic bath can be made without departing from the invention. Therefore, the specific example given above should be considered as illustrative only and not as limiting the scope of the following claims.

I claim:

1. A process of removing copper plate from a copper plated part comprising subjecting said part as an anode to electrolysis in an electrolyte consisting of an aqueous solution of ammonium hydroxide and ammonium nitrate.

2. A process for removing copper from a copper-plated part comprising subjecting said part as an anode to electrolysis at a current density of between about 20 and 30 amperes per square foot in a bath consisting of about 12% to 15% by weight of ammonium nitrate and about 5% to 10% by volume of 26 Baum ammonium hydroxide and the remainder water.

3. A process for removing copper from a copper-plated part comprising subjecting said part as an anode to electrolysis at a current density of about 25 amperes per square foot in an electrolytic bath consisting of about 15% by weight of ammonium nitrate and about 10% by volume 

